Heterodyne direction finder with single receiver



Sept. 24, 1946. W, H WIRKLER 2,408,122

HETERODYNE DIRECTION FINDER WITH SINGLE RECEIVER Filed Nov. l5, 1940 2 Sheets-Sheet l /f/ AZZ v sept. 24, 1946.

W. H. WIRKLER HETERODYNE DIRECTION FINDER WITH SINGLE RECEIVER Filed NOV. l5, 1940 2 Sheets-Sheet 2 @AIVD PA SS ANPL/F/ER AMPLIFIER INVENTon Waffe/p 3E- WM5/eww,

Patented Slept. 24, 1946 HETERODYNE DIRECTION FINDER WITH SINGLE RECEIVER Walter H. Wirkler, Cedar Rapids, Iowa, assigner to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application November 15, 1940. Serial No.. 365.826

(Cl. Z50- 11) 16 Claims. 1

This invention relates broadly to radio direction finders, and more specifically to direction finders of the heterodyne ty-pe in which the indication is based on the phase relationship between audio frequency modulation products of a. locally generated wave radiated from an injection antenna, and a signal wave arriving from a distance.

One object of this invention is to provide an indication of the relative phase of the received signal voltage in two spaced antennas by means of a single receiver alternately connected to each antenna at a switching frequency which is higher than the -low lfrequency modulation product resulting from the heterodyning action between the distant signal and the-local oscillator.

Another1 object of thisv invention is to provide automatic differential gain control action between the two antenna circuits, based on the presence `of switching frequency components. in the output of a single receiver channel.

A further object of my invention is to provide a heterodyne radio direction finding system employing a single radio receiver, alternately connected with separate receiving antennas, and a novel arrangement for providing currents of cornparable lpha-se for determining phase differences indicative of direction.

Still another object ofmy invention is t0 provide a heterodyne radio direction finding system employing a single radio receiver, electronically coupled alternately withseparate receiving antennas at a predetermined switching frequency, and an inte-rmodulaton arrangement involving components of the switching frequency and the heterodyne beat frequency for providing a current proportional to the phase dilerence between the instantaneous currents from the receiver, for indicating direction.

Other and further objects of my invention reside in the system and circuit arrangements hereinafter described with reference to the accompanying drawings in which:

Figure 1 is a block diagram illustratingT the basic arrangement of elements in the system of my invention; Fig. 2 is a schematicdiagram of one embodiment of my invention, following the arrangement of Fig. 1 and incorporating additional features, such as the automatic dinerential volume control, for improved operation; and Fig. 3 comprises theoretical wave form diagrams of voltages in different portions of one circuit connected with the output of the receiver, for directional indications. I

Direction nders of the heterodyne type have certain inherent advantages with respect tophase shift in independently tuned antenna circuits .which have been completely described in my 'copending application YSerial No. 294,522, filed September l2, 1939, for Radio direction finding system, and in a number of related applications. lt is the object of this invention to -retain the advantages of the heterodyne method while using only a single receiver channel Vinstead of a separate receiver channel for each antenna circuit asin the cases mentioned above. Direction linders not of the heterodyne type commonly use a single receiver channel cooperatively energized by two receiving antennas Whichare connected directly to the receiver at all times, although some use a single receiver alternately switched tothe two receiving antenna circuits. Since the switching frequency in this arrangement must, of necessity, be much lower than the signal frequency, the indication of phase difference between currents in the two antenna circuits -is effective during only a small fraction of the total number of l signal frequency cycles, and the directional sensitivity of such a device is relatively poor in terms of signal-to-noise ratio.

The-signal vfrequency in the system of my invention is reduced on the heterodyne principle to a beat frequency 'with respect to which the switching frequency is .made relatively high, for example, abeat frequency of 250 cycles may be employed with a switching frequency of 3G00 cycles; the theoretical wave form diagrams of Figs. 3 and 4 are drawn accordingly. The frequencies must be always sufficiently different for separation in filters, the switching rfrequency lbeing invariably greater than the beat frequency.

In Fig. l, I have indicated a local oscillator l differing in frequency from vthe signal frequency by a low audio beat frequency, an injection antenna 2 for inducing equally into. receiving antenna circuits 3 and v4 vvoltages at the local oscillator frequency, and switching-means at '5, (6), which alternately supply theinput of a receiver 8 with energy from antenna circuits 3 and '13; The antennas are switched alternately by means of switching frequency voltage from switching oscillator l. The audio output voltage from receiver 8 is supplied through frequency selective network 9 to a demodulator or rectifier lil. Network 9 selects modulation components in a frequency band centered on the switching frequency of 3,000 cycles per seco-nd. Rectifier lll is supplied also with switchingfrequency voltage directly from oscillator l. The output of rectifier IU, as it appears at phase shifting network f4,

will contain voltage of beat frequency when a phase difference exists between the signal frequency voltages at antenna circuits 3 and 4 as explained below. The audio output voltage from receiver 8 is also supplied to frequency selective network II which selects currents of beat frequency for application with the output of the rectifier II, in 'proper phase, to the balanced rectifier I6 for deriving an indicating voltage.

In Fig. 3, at I, curve fb represents the audio output of receiver 8 when the receiver is continuously connected to antenna 3 for example. Curve fb' represents the output of receiver 8 when its input is continuously connected to antenna 4. Curves fb and fb have been drawn with a slight phase displacement such as would exist when the signal is arriving from the direction different from a perpendicular to a line joining the two antennas. Under the action of switching voltage from oscillator 'I, the input of receiver Il will be connected alternately to the two antennas so that the audio output voltage from receiver Il may be expected to look something like the curve X, the solid line in the group at I, Fig. 3. Network I I in selecting currents of beat frequency delivers an output wave A which is substantially the average of the curve X; the curve fA is shown in full line at II, Fig. 3.

Network 9 in selecting sideband components around the switching frequency can be thought of as passing an alternating current of wave form somewhat similar to the curve X' at III, Fig. 3. Rectifier IB can be thought of as a reversing switch operating synchronously with switching voltage alternations from 1. Hence, the wave at III, Fig. 3, when rectified n III, might be expected to look something like the curve fia at IV, Fig. 3. Curve B at IV is seen to contain a component of beat frequency displaced approximately 90 degrees in time phase from the beat frequency component fA directly obtained from the output of the receiver. This beat frequency component B from rectifier IU is therefore passed through phase shifting network I4, in order that the component JB may be applied to rectifier I6, Fig. 1, in phase with the beat frequency component fA received from the output of receiver 8 through low pass filter II. The output of the rectifier I6 is applied to indicating means Vat I'I. Rectifier I6 will produce D.C. output voltage proportional to the phase differences between the signal voltages in antennas 3 and 4 as evidenced by the beat frequency component fe, and of polarity indicating the sense of this phase difference as evivdenced by the polarity of the fe component with respect to that of the fA component.

In Fig. 1, I have indicated the high frequency signal and heterodyne waves as Fs and Fh, respectively, with Fh components being supplied to receiver 8 with Fs components from antenna 3 and Fs components from antenna 4, alternately. In the receiver 3, the primary function is the mixing of the signal and heterodyne components to produce beat frequencies fb and fb', corresponding in phase to the signal components FB and Fs', respectively. A t the same time, receiver 8 operates to produce beat frequency components as sidebands of the switching frequency. In the output of receiver 8, filter II averages the primary beat frequency waves fb and fb and passes the resultant wave fA to the balanced rectifier I6. Filter 9, also connected with the output ofthe receiver, passes the sideband components centered on the switching frequency f, the output being as represented at III, Fig. 3. After demodulation at I with the aid of current from the switching oscillator, the wave of beat frequency je is established as at IV, Fig. 3, the phase of which is of particular interest. From Fig. 3, it will be noted that the peak y of the fe wave coincides in phase with the intermediate nodal point z between the phase displaced waves of beat frequency fb and fb', which corresponds to the actual node z' of the average wave fA, wherefore the fB Wave is 90 out of phase with the average Wave A of beat frequency. Through phase shifting means I4, the resultant Wave of beat frequency ,B is shifted substantially 90 in phase, whereupon this component of beat frequency je and the component f. delivered from filter I I are applied to the balanced rectifier I6 lessentially in phase, and a direct current voltage is obtained proportional to the amplitude of the fe component and of polarity corresponding to the sense of je as referred to fA, the average or composite wave of beat frequency.

The significant component fB is zero when the directional components fb and fb' are in like phase, and there is then no voltage output from rectifier I6. The component JB appears upon the occurrence of any out of phase relation of components fb and fb. and the output of rectifier I6 assumes a value proportional to the out of phase relation. It might be again noted that low pass filter II, operating at the low beat frequency, receives portions of the beat-frequency waves fb and fb alternately at the switching frequency, and effectively alleviates the pulsating character of the waves so that a substantially continuous, composite wave of beat frequency is delivered for coaction in the rectifier I6 with the continuous wave fe derived from both the fb and fb components through the rectifier I0.

Referring now to Fig. 2, the same general circuit arrangement shown in Fig. 1 is evident in the provision of antennas 2, 3 and 4 coupled respectively to a source of injection energy Ia-Ic and switching means 5 and 6; switching oscillator l, which may be of any suitable form; receiver 8, shown as of the superheterodyne type; filters 9 and II; rectiers I0 and I6; phase shifting means at I4, which is supplemented by phase shifting means I2 in the connection from lter II; and the indicating means II. The switching means are disclosed as multi-grid electron tubes biased to cut-off by a source of potential 23 but alternately operable under the action of the control voltage from switching oscillator 'I to energize the receiver. The rectifier I0 is of the doubly 55 balanced or ring type, and is supplied with components of switching frequency from oscillator I in phase with the components of like frequency delivered from the band pass lter 9.

sideband components of the beat frequency (IV 60 in Fig. 3), are thus detected and supplied, through phase shifting means I4 and such amplification means at I5 as may be required, to the rectifier I6 which is also of the doubly balanced or ring type.

The low pass filter II is connected with rectifier I6 through phase shifting means I2 and such amplification means at I3 as may be required. The phase shifting means I2 and I4 are cornplementary in character, and together effect the 70 substantially 90 phase adjustment of the beat frequency component ,fB with respect to the composite average wave fA. Rectifier I6 operates at the beat frequency and delivers direct current for operation of the meter means shown at I1 as a 75 result of an out of phase relationship in the beat frequency components fb and fb', evidenced through JB and corresponding to a like relationship in the signal components Fs and Fs', as hereinbefore explained, for indicating distance.

Rectifier I6 depends for operation `upon beat frequency current of Vpropery phase character from rectifier I8 through network `Ill. This current (fe) is present only when there is -a phase inequality between the lcurrents fb and fb'. Hence the zero-center D.C. meter indicator Il shows a deflection only when the antenna system is rotated olf bearing, the sense of said indication showing the directionof 'this off-bearing rotation. The meter shown at VI-'Irepresents one type of indicating means adaptable to the system of my invention, but it will be understood that oscilloscopio `or other Vequivalent vmeans may be employed, as desired.

In the direction finder here described, the two bea'tfrequency components appliedto I6 are-obtained effectively vthrough different channels. That applied to amplifier-I3, for example, is directly the output voltage from the receiver; while that obtained from vl is passedthrough thereceiver as sidebands around the switching frequency before being converted tocurrent of beat frequency. The result'is that the relative phase delay of these two currents is rather indeterminate and it is essential that a'rather -high order of amplitude equality 'exist' between signal cur- VVare provided. The output ofre'ceiver 8 will contain components of `switchingfrequency depending 'on the amplitude'difference between carrier or Fh currents from 3 and 4, Yas `'distinguished from the sideband components around the switching frequency which indicate phase difference between V`the two signal currents. The switching frequency components resulting from amplitude difference are passed through a narrow band pass lter I8 to 'rectifier I9; 'Rectifier I9 also receives switching frequency components directly from oscillator 1, through phase shifting 'network '20. The result vis that a `direct voltage of suitable polarity is obtained from the output of i9 and applied as gain 'control voltage to tube '6 through lead 2l, the arrangement including blocking condenser '22., Ajl'iigh order of differential gain control can b'e'obtaned byinserting an audio frequency amplifier 'between VI8 'and I9.

VNo modulation sidebandsia're found in the rectif-ler lfd as in the rectifier l0, even when the signalfcomponents are out of phase, because the band width of filter I8 is limited so vthat'only components of switching frequency may pass, and accordingly the output of Vrectifier 'I9 is a direct current resulting from rectification of carrier energy, as distinguished from the alternating'current of beat frequency present as a sideband and detected in rectifier I0. 'That is,'whe`n an amplitude difference exists between the carrier voltages received from the two antennacircuits, the D.C. output from-the receiver iiuctu'ates in synchronism with the switching operation. 'Rectifier i9, considered as a synchronously Voperating 'reversing switch, will then have in its output a D.C. component, the polarity of 'which Vdepends on which of the two antenna circuits delivers the stronger carrier voltage. Such D.C. Voltage will be suitable for the differential gain correcting function operating on the grid bias voltage of switching tube'G.

I prefer the arrangementfsliownrforobtaining y 'current to be introduced into ection l antenna 2. By this arrangement, the local injected frequency fas it :appears in the intermediate frequency channel of the receiver will lbeconstant and independent of the .receiver tuning, andhence 5 its phase delay-and the envelope phase delay for sidebands around this locally introduced frequency--will be constant Aas far as the intermediatefrequencies are concerned, and the adjustment of the phase shifting network will vbe more nearly independent of receiver tuning.

This is effected, as shown in Fig. 2, by employing-a mixer circuit Ic to feed the injection antenna -2, and supplying the mixer lc with energy from oscillator la of fixed frequency andoscillator lb which is also the conversion oscillator for the superheterodyne receiver 8. Oscillator la operates at a frequency differing from the intermediate frequency of the receiver by the beat frequency fb. The frequency of the energy supplied to'tlie linjector loop has a frequency Fh differing from the signal frequency Fs bythe desired beat frequency over the tuning range, and its intermediate frequency remains constant, due to the interrelation of the conversion frequency `of-'oscillator-'lb, as-disclosed.

While I have disclosed my invention in certain general and preferred embodiments, various modifications may be made therein and I desire it understood thatno limitations upon my invention are intended thereby `but only by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

i.. In a heterodyne radio direction finding sys- Si; tem, incombination: a direction'a-lantenna system,inc'luding a'pair of signal receiving antennae and a locally energized injector antenna coupled -in like relation to both said receivingantennae; a receiving circuit including detector means; `switching'means for connecting said receiving Vcircuit alternately with each of said receiving antermae for producing heterodyne output beat frequency components, derived in said detector imeans from receivedsignal energy and energy 40 fronrsaid injector antenna, and varying in phase relation as the signals received at the respective receiving antennae; said beat frequency components being interrupted at the frequency of said switching means and constituting amplitude 00 modulation sidebands'of the switching frequency proportional in amplitude to the phase relation of lsaid beat frequency components; means for detecting said modulation sidebands for producing a beat frequency-current proportional in amplitude to the phase relation of said received signals; said beat frequency components, pulsating at said switching frequency, comprising together a continuous wave constituting an average beat frequency `current of effectively'iixed phase; balanced rectifier means cooperatively energized by both said beat frequency currents in substantially like phase for producing a direct voltage proportional-to the'phase relation of said received signals, and direct-ion indicating means energized D by'said direct voltage.

' 2. In a heterodyne radio direction finding system, the combination set forth in claim l wherein the switching frequency is greater than said beat frequency, and including filter means between said receiving circuit Yand the means for detecting said modulation sidebandssaid filter means being tuned vto pass sideband energycentered onsaid switching frequency, and additional 'filter meansconnectedwiththe output of. said re- 'ieeiving 'nlroei-tirer :selectively `passing and integrating the components of beat frequency comprising said average beat frequency current of effectively fixed phase.

3. In a heterodyne radio direction finding system, the combination set forth in claim 1 wherein the detected beat frequency current and the average beat frequency current, as produced, are in phase quadrature; and including phase shifting means for aligning .said currents in like phase for cooperatively energizing said balanced rectifier means.

ll. In a heterodyne radio direction finding system, the combination set forth in claim 1 wherein said switching means includes an electron tube amplifier in circuit with each receiving antenna, and a control oscillator connected with each electronf tube amplifier for blocking the transfer of energy alternately from the respective antennae.

5. In a heterodyneradio direction finding system, the combination set forth in claim l'wherein said switching means includes an electron tube amplifier in circuit with each receiving antenna, and a cont-rol oscillator connected with each electron tube amplifier for blocking the transfer of energy alternately from the respective antennae; said means for detecting the modulation sidebands of said switching frequency having a connection with said control oscillator for receiving current of switching frequency from said oscilla-- tor in proper phase for detecting said modulation sidebands.

6. In a heterodyne radio direction finding system, the combination set forth in claim 1 wherein said switching means includes an electron tube amplifier in circuit with each receiving antenna, and a control oscillator connected with each electron tube amplifier for blocking the transfer of energy alternately from the respective antennae; and including in said combination filter means connected with the output of said receiving circuit for selectively passing components of switching frequency resulting from difference in amplitude of carrier energy in the input of said receiving circuit, means for rectifying said components of switching frequency for producing a direct voltage proportional to said amplitude difference, and means for controlling the gain in one said electron tube amplifier in accordance with the magnitude of the last said direct voltage 7. In a heterodyne radio direction nding system, in combination: a directional antenna system, including a pair of signal receiving antennae and a locally energized injector antenna coupled in like relation to both said receiving antennae; a receiving circuit including detector means; switching means for connecting said receiving circuit alternately with each of said receiving antennae, for producing heterodyne output beat frequency components varying in phase relation as the signals received at the respective receiving antennae; means for deriving from said beat frequency components, interrupted at the switching frequency, a beat frequency current proportional in amplitude to the phase relation of said received signals; means for integrating said beat frequency components, pulsating at the switching frequency, to produce an average beat frequency current of fixed phase; balanced rectier means cooperatively energized by both said beat frequency currents in like phase for producing a direct voltage proportional to the phase relation of said received signals and direction indicating means energized by said direct voltage.

8. In a heterodyneradio direction nding sysi tem wherein directional indications are dependent upon the phase relation of two independent signal currents, means including a single receiving circuit for providing components of said `currents Vin alternate periods at` a rate substantially higher than the frequency of said currents; means for integrating said components, to produce an average current of fixed phase; means for detecting the differential of successive components, to produce a'separate current proportional in amplitude to the phase relation of said two independent signal currents; and means cooperatively energized |by said average current and said separate current, in like phase, for producing a direct voltage proportional to the phase relation of said independent signal currents,for directional indications; the polarity of said direct voltage being dependenton the sense of the differential of the successive components, as detected relative to the fixed phase of said average current.

9. In a heterodyne radio direction finding system wherein directional indications are dependent upon the phase relation of two independent signal currents, means for producing said currents as impulses occurring in successive periods at a frequency substantially greater than that of said currents, means for integrating said impulses to produce an average ycurrent of fixed phase, means for detecting the differential of successive impulses to produce a separate current proportionalin amplitude to 'thephase relation of said two independent signal currents, and means cooperatively energized by said average current and said separate current in like phase for producing a direct voltage proportional to the phase relation of said independent signal currents and of a polarity dependent upon the sense of the differential of the successive impulses as detected with respect to the fixed phase of said average current, for directional indications.

10. In a heterodyne radio direction finding system wherein directional indications are dependent upon the phase relation of two independent Signal currents, in combination, meanscfor producing said currents as impulses occurring in successive periods at a frequency substantially greater than that of said currents', means for deriving from said impulses a first substantially continuous current o f fixed phase as the average of said current impulses and a second substantially continuous current dependent upon the phase relation of said independent signal currents in amplitude and polarity, and means for indicating the amplitude of said second current, and its polarity in reference to the polarity of said first current of xed phase, for determining direction.

ll. In a heterodyne radio direction finding system, the combination set forth in claim l0 and including automatic differential gain vcontrol means operative to maintain said independent signal currents equal rin amplitude prior to the operation of the first said means.

12. In a heterodyne radio direction nding systern, in combination: a directional antenna system` including a pair of signal receiving antennae and a locally energized injector antenna coupled "in likerelation to both said receiving antennae;

areceiving circuit including detector means; switching means for connecting said receiving circuit alternately with eachvof said receiving v antennae for producing independent heterodyne output-.beat frequency currents as impulses occurring in successive periods at said switching .,frequency, said currents being derived in said :detectormeans from received signal energy and energy from said injector antenna and varying in phase relation as the signals received at the respective receiving antennae; means for integrating said impulses to produce an average current of xed phase; means for detecting the differential of successive impulses to produce a separate current proportional in amplitude to the phase relation of said independent heterodyne beat frequency currents; balanced rectier means cooperatively energized by said average current and said separate current in like phase for producing a direct voltage proportional to the phase relation of said received signals; and direction indicating means energized by said direct Voltage.

13. In a heterodyne radio direction nding system, the combination set forth in claim 12 wherein said switching means includes an electron tube amplifier in circuit with each receiving antenna, and a, control oscillator connected with each electron tube ampliiier for blocking the transfer of energy alternately from the respective antennae.

14. In a heterodyne radio direction nding system, the combination set forth in claim 12 Wherein said switching means includes an electron tube amplifier in circuit with each receiving antenna, and a control oscillator connected with each electron tube amplifier for blocking the transfer of energy alternately from the respective antennae; and including in said combination separate means for detecting the diierential of rectified carrier energy in successive periods at the output of said 1G receiving circuit for producing a direct voltage proportional to said diiferential, and means for controlling the gain in one said electron tube amplier in accordance with the magnitude of the last said direct voltage.

15. In a heterodyne radio direction finding system wherein directional indications are dependent upon the phase relation of two independent signal currents, in combination, means for producing said currents as impulses occurring in successive periods at a frequency substantially greater than that of said currents, means for deriving from said impulses a substantially `continuous current dependent upon the phase relation of said independent signal currents in amplitude and polarity, means for combining said successive impulses for producing a substantially continuous wave of effectively xed phase, and means for indicating the amplitude of said continuous current, and its polarity in reference to the polarity of said continuous Wave of eifectively nxed phase, for determining direction.

16. In a heterodyne radio direction nnding system, the combination set forth in claim 10 including means for producing phase quadrature relation of the first and second currents, and phase shifting means for aligning the currents in like phase for cooperatively energizing the indieating means.

WALTER H. WIRKLER. 

